Children 5-9 years-old and newly diagnosed with type 1 diabetes mellitus (T1DM) have an increased lifetime risk of developing end stage renal disease and poor glycemic control may exacerbate this risk. Our pilot data suggest that up to 44% of 5-9 year-olds new to T1DM experience declining glycemic control early in their disease despite increasing treatment intensification as they exit the honeymoon period. Moreover, from the literature, we can ascertain that these declines may not be solely due to a loss in C peptide production and may be an outcome of nonadherence to daily T1DM management. Targeted behavioral interventions can help patients to achieve better adherence and glycemic control. Unfortunately, no such interventions exist for families of 5-9 year-olds and there is a lack of foundational research to inform intervention development for these families. The PI is a leader in studying T1DM management in young children. With the revised R01, she now seeks to expand her research to focus on intervention development for children in middle childhood. The new R01 aims are to 1) prospectively identify adherence and glycemic trajectories in families of 5-9 year-olds old with new-onset T1DM; 2) identify modifiable parent, child, and family behavior factors that serve as barriers or facilitators of optimal adherence and glycemic trajectories in these families; 3) develop a prototype preventative intervention specifically tailored to these families. The revised R01 aims will be accomplished in two studies. In Study 1, 120 families will complete a 30-month prospective, longitudinal study of adherence and glycemic trajectories and the causes and consequences of these in 5-9 year-olds new to T1DM. In Study 2, 28 parents will participate in serial semi-structured interviews designed to gather new data to inform the content of our preventative intervention and review and refine each intervention module to ensure a high level of acceptability and engagement among families. Finally, 12 families will participate in a pilot feasibility study of the intervention. This R01 is significant because we expect it to produce the necessary data to inform the development of a specific and targeted behavioral intervention which may ultimately result in improved trajectories of glycemic control and adherence for 5-9 year-olds with new-onset T1DM. It is innovative because it proposes a mixed methods research design to efficiently inform our interventions and increase the likelihood our treatments will be targeted to the unique characteristics, needs, and behavioral patterns of 5-9 year-olds with T1DM. We also believe it is innovative in its plan to recruit a homogeneous (by age) sample of children and Study 1's prospective, longitudinal design, which both represent departures from existing research in youths with new-onset T1DM. The research team has complementary expertise in treatment development, longitudinal research, health behavior theories, and T1DM management. The expected outcomes are a relevant and empirically-based preventative treatment, identification of a creditable attention control treatment, and novel longitudinal data regarding health outcomes for 5-9 year-olds with new-onset T1DM.